9.2 Climate and Climate Change

Chapter 1 discusses the nature of the climate system
and the climate variability and change it may undergo, both naturally and as
a consequence of human activity. The projections of future climate change discussed
in this chapter are obtained using climate models in which changes in atmospheric
composition are specified. The models "translate" these changes in
composition into changes in climate based on the physical processes governing
the climate system as represented in the models. The simulated climate change
depends, therefore, on projected changes in emissions, the changes in atmospheric
greenhouse gas and particulate (aerosol) concentrations that result, and the
manner in which the models respond to these changes. The response of the climate
system to a given change in forcing is broadly characterised by its "climate
sensitivity". Since the climate system requires many years to come into
equilibrium with a change in forcing, there remains a "commitment"
to further climate change even if the forcing itself ceases to change.

Observations of the climate system and the output of models are a combination
of a forced climate change "signal" and internally generated natural
variability which, because it is random and unpredictable on long climate time-scales,
is characterised as climate "noise". The availability of multiple
simulations from a given model with the same forcing, and of simulations from
many models with similar forcing, allows ensemble methods to be used to better
characterise projected climate change and the agreement or disagreement (a measure
of reliability) of model results.

9.2.1 Climate Forcing and Climate Response

The heat balance
Broad aspects of global mean temperature change may be illustrated using a simple
representation of the heat budget of the climate system expressed as:

dH/dt = F - T.

Here F is the radiative forcing change as discussed in Chapter
6; aT represents the net effect of processes acting to counteract changes
in mean surface temperature, and dH/dt is the rate of heat storage in the system.
All terms are differences from unperturbed equilibrium climate values. A positive
forcing will act to increase the surface temperature and the magnitude of the
resulting increase will depend on the strength of the feedbacks measured by
T. If
is large, the temperature change needed to balance a given change in forcing
is small and vice versa. The result will also depend on the rate of heat storage
which is dominated by the ocean so that dH/dt = dHo/dt = Fo
where Ho is the ocean heat content and Fo is the flux
of heat into the ocean. With this approximation the heat budget becomes F =
T + Fo, indicating
that both the feedback term and the flux into the ocean act to balance the radiative
forcing for non-equilibrium conditions.

Radiative forcing in climate models
A radiative forcing change, symbolised by F above, can result from changes in
greenhouse gas concentrations and aerosol loading in the atmosphere. The calculation
of F is discussed in Chapter 6 where a new estimate of
CO2 radiative forcing is given which is smaller than the value in
the SAR. According to Section 6.3.1, the lower value is
due mainly to the fact that stratospheric temperature adjustment was not included
in the (previous) estimates given for the forcing change. It is important to
note that this new radiative forcing estimate does not affect the climate change
and equilibrium climate sensitivity calculations made with general circulation
models. The effect of a change in greenhouse gas concentration and/or aerosol
loading in a general circulation model (GCM) is calculated internally and interactively
based on, and in turn affecting, the three dimensional state of the atmosphere.
In particular, the stratospheric temperature responds to changes in radiative
fluxes due to changes in CO2 concentration and the GCM calculation
includes this effect.

Equivalent CO2
The radiative effects of the major greenhouse gases which are well-mixed throughout
the atmosphere are often represented in GCMs by an "equivalent" CO2
concentration, namely the CO2 concentration that gives a radiative
forcing equal to the sum of the forcings for the individual greenhouse gases.
When used in simulations of forced climate change, the increase in "equivalent
CO2" will be larger than that of CO2 by itself, since
it also accounts for the radiative effects of other gases.

1%/yr increasing CO2
A common standardised forcing scenario specifies atmospheric CO2
to increase at a rate of 1%/year compound until the concentration doubles (or
quadruples) and is then held constant. The CO2 content of the atmosphere
has not, and likely will not, increase at this rate (let alone suddenly remain
constant at twice or four times an initial value). If regarded as a proxy for
all greenhouse gases, however, an "equivalent CO2" increase
of 1%/yr does give a forcing within the range of the SRES scenarios.

This forcing prescription is used to illustrate and to quantify aspects of
AOGCM behaviour and provides the basis for the analysis and intercomparison
of modelled responses to a specified forcing change (e.g., in the SAR and the
CMIP2 intercomparison). The resulting information is also used to calibrate
simpler models which may then be employed to investigate a broad range of forcing
scenarios as is done in Section 9.3.3. Figure
9.1 illustrates the global mean temperature evolution for this standardised
forcing in a simple illustrative example with no exchange with the deep ocean
(the green curves) and for a full coupled AOGCM (the red curves). The diagram
also illustrates the transient climate response, climate sensitivity and warming
commitment.